The redundancy of the ribosomal RNA cistrons is reduced by the addition of Bromodeoxyuridine to larval stages of Drosophila. We intend to characterize the nucleotide sequences and tissues in which the effects occur, determine if the drug must be incorporated into DNA to be effective, and relate the effects to the functional status of the ribosomal RNA cistrons. In addition, we will determine if BrdU interacts with compensatory and magnification events which are known to influence the redundancy of the ribosomal DNA.